I saw this JavaScript error earlier this week, and thought I’d write a few lines about it. It appears as you try to submit a form created with the EPiServer Forms functionality, that likely has custom validators on some of it’s fields.

As EPiServer Forms tries to run it’s client side validation on the fields it gets presented with one requiring a custom validator; which it cannot find in it’s array of available validators. The value of f.Type in the image above will likely be the fully qualified name of your custom validator (that’s the namespace plus the class name of your validator class in the solution); like for instance MyProject.Core.Forms.Validators.LuhnValidator.

If we didn’t register a client side validator with this name, the variable e won’t be set, hence it will not be a function in the statement below. This will cause the typeof e.validate to fail as we cannot read property ‘validate’ of undefined.